Hydrogen production

ABSTRACT

A process for manufacturing high pressure, high purity hydrogen which comprises (a) generating at a pressure below about 450 psig a hydrogen-rich gas containing sufficient Co2 so that the molecular weight of the hydrogen-rich gas is at least four; (b) centrifugally compressing the hydrogen-rich gas from a pressure below about 450 psig to a substantially higher pressure above 450 psig to obtain high pressure hydrogen-rich gas; (c) removing CO2 from the high pressure hydrogen-rich gas to obtain high purity hydrogen, at least part of the CO2 being removed by absorbing CO2 in a mixed absorbent comprising a chemical absorbent and a physical absorbent.

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States Patent Smith et al.

[ HYDROGEN PRODUCTION 3,532,467 10/1970 Smith et al ..23/212 R [75] Inventors: Calvin S. Smith; William J. McLeod, 2;222:22 both of El Cemto 3,576,603 4 1971 Smith et al..... ....23/212 R Assigneez Chevron Research Company San et al. Francisco, Calif 3,618,331 11 1971 Smith et al. ..62/11 [22] Filed: 1971 Primary Examiner-Edward Stern 21 AppL 554 AttorneyGeorge F. Magdeburger, Roy H. Davies,

Thomas G. DeJonghe and J. A. Buchanan, Jr. Related [1.8. Application Data [63] Continuation-impart of Ser. No. 736,520, May l7, [57] ABSTRACT 1968, Pat which is a cominuafion-in- A process for manufacturing high pressure, high purity part of Ser. No. 665,106, Sept. 1, l967,abandoned. hydrogen which comprises (a) generating at a pres sure below about 450 psig a hydrogen-rich gas con- [52] US. Cl. ..423/650, 423/226, 423/228, taming sufficiem CO2 50 that the molecular weight of 423/232 423/655 208/107 252/373 the hydrogen-rich gas is at least four; (b) centrifugally 2 "fi 1/02 Colb 1/16 q 1/32 compressing the hydrogen-rich gas from a pressure 1 2 2 3 /2 208/108 below about 450 psig to a substantially higher pressure /1 g g igj fg 23 above 450 psig to obtain high pressure hydrogen-rich l 52/373 gas; (c) removing CO from the high pressure hydrogen-rich gas to obtain high purity hydrogen, at [56] References cued least part of the CO being removed by absorbing CO UNITED STATES PATENTS in a mixed absorbent comprising a chemical absorbent and a physlcal absorbent. 3,347,621 /1967 Papadopoulos et al.... ..1..23/2l0 X 3,401,111 9/l968 Jackson ..208/108 13 Claims, 1 Drawing Figure H 1 c H RECYCLE Hz PURITY 1-12 HYDROCARBON FEED WATER 55 HYDROCONVERSION ZONE g I0 5 11/ 62 TO E-6 8 4a 6l\ 0 25 I PARTIAL BO'LERS 69 TURBINE ANTIFREEZE 4r OXIDATION DRWER ,9 3 OR STEAM R CO REFORMING 2/ 22 2a 24 C02 0 2 AND H25 (INCLUDING ABSORBER SHIFT S R N) CENTRIFUGAL ZONE COMPRESSOR 2 z COOLER t) o '2 2 a: a z an [8 z a: o v 3 a 2 12 C02 OR CO2 :5 1: m" I AND HzS u w 0 1r 2 O I Q m O o g z N g 1/: w 7/ 0 a: o m O 5 70 SEPARATOR 15 1: 0 o P D: g N WATER TO 8 DEAERATOR ABSORBENT REGENERATOR STEAM aa E-B REBOILE 4a HYDROGEN PRODUCTION CROSS REFERENCES This application is a continuation-in-part of application Ser. No. 736,520, titled Hydrogen Manufacture Using Centrifugal Compressors, filed May 17, 1968, now U.S. Pat. No. 3,618,331, patented Nov. 9, 1971, which in turn is a continuation-in-part of application Ser. No. 665,106, filed Sept. 1, 1967, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to processes for the production, compression, and purification of gases; and, more particularly, it relates to a process for supplying high pressure, high purity hydrogen gas at elevated pressure. In a still more particular aspect, the invention relates to a process for obtaining high pressure, high purity hydrogen for use in a hydroconversion process. By hydroconversion process is meant a process wherein hydrogen is reacted with hydrocarbons so as to convert the hydrocarbons to more desirable hydrocarbons or hydrocarbon products.

2. Description of the Prior Art A. Means for obtaining raw, hydrogen-rich gas There are a number of current processes available for the production of raw hydrogen. Many of these processes use hydrocarbons as a source of hydrogen. Two of the most widely practiced methods of obtaining raw, hydrogen-rich gas are steam reforming and partial oxidation.

In typical steam reforming processes, hydrocarbon feed is pretreated to remove sulfur compounds which are poisons to the reforming catalyst. The desulfurized feed is mixed with steam and then is passed through tubes containing a nickel catalyst. While passing through the catalyst-filled tubes most of the hydrocarbons react with steam to form hydrogen and carbon oxides. The tubes containing the catalyst are located in a reforming furnace, which furnace heats the reactants in the tubes to temperatures of 1,2001,700 F. Pressures maintained in the reforming furnace tubes range from atmospheric to 450 psig. If a secondary reforming furnace or reactor is employed, pressures used for reforming may be as high as 450 psig to 700 psig. In secondary reformer reactors, part of the hydrocarbons in the effluent from the primary reformer is burned with oxygen. Because of the added expense, secondary reformers are generally not used in hydrogen manufacture but are used where it is desirable to obtain a mixture of H and N as in ammonia manufacture. The basic reactions in the steam reforming process are:

Because the hydrogen product is used in high-pressure processes, it is advantageous to operate at high pressure to avoid high compression requirements. However, high pressures are adverse to the equilibrium; and

higher temperatures must be employed. Consistent with hydrogen purity requirement of about to 97 volume percent I-1 in the final H product and present metallurgical limitations, generally the single stage reforming is limited commercially to about 1,550F. and 300 psig.

In typical partial oxidation processes, a hydrocarbon is reacted with oxygen to yield hydrogen and CO. Insufficient oxygen for complete combustion is used. The reaction may be carried out with gaseous hydrocarbons or liquid or solid hydrocarbons; for example, with methane, the reaction is:

With heavier hydrocarbons, the reaction may be represented as follows:

Both catalytic and noncatalytic partial oxidation processes are in use. Suitable operating conditions include temperatures from 2,000F. up to about 3,200F., and pressures up to about 1,200 psig, but generally pressures between 100 and 600 psig are used. Various specific partial oxidation processes are commercially available, such as the Shell Gasification Process, Fauser-Montecatini Process, and the Texaco Partial Oxidation Process.

There is substantial CO in the hydrogen-rich gas generated by either reforming or partial oxidation. To convert the CO to H and CO one or more CO shift conversion stages are typically employed. The CO shift conversion reaction is:

This reaction is typically effected by passing the CO and H 0 over a catalyst such as iron oxide activated with chromium. The reaction kinetics are faster at higher temperature, but the equilibrium to hydrogen is favored by lower temperatures. Therefore, it is not uncommon to have a high temperature shift stage followed by a low temperature shift stage. Pressure has little bearing on the equilibrium in the water-gas shift reaction.

B. CO or CO +H S removal Because most hydrogen using processes, particularly hydroconversion processes, operate more efficiently with high purity hydrogen, it is generally required to remove impurities, such as CO from the raw hydrogen generated in the hydrogen plant before the hydrogen is passed to the hydrogen-using process. Perhaps the most widespread method of removing CO from other gases is the absorption of CO in an alkanolamine, such as diethanolamine (DEA) or monoethanolamine (MEA). Largely because of its relatively low molecular weight, MEA is generally the preferred absorbent of the alkanolamines. The CO forms a loose chemical bond with the amine when it is absorbed.

In using any of the commonly used alkanolamine absorbents, an absorber and stripper are typically arranged in a figure eight process configuration. The CO -containing gas is fed into the bottom of the absorber where CO is absorbed in downward flowing absorbent. Purified gas with the CO removed leaves the top of the absorber. Rich absorbent from the bottom of the absorber is passed to the top of a stripping column where it is regenerated as it passes from the top to the bottom of the stripping column. The regenerated absorbent passes from the bottom of the stripper to the top of the absorber to complete the figure eight path of the absorbent as it flows down through the absorber trays, or packing material, absorbing CO A large amount of heat is required to strip the CO from the MEA absorbent which is typically used because of the chemical bond that occurs between the CO and the MEA. For instance, in a large hydrogen plant producing 135 X standard cubic feet per day of hydrogen, over 300 X 10 BTU s per hour are generally required to reboil the MBA in order to effect the regeneration of the MEA. These 300 X 10 BTUs per hour are equivalent to over 1,000,000 dollars per year in terms of steam (at a value of about 40 cents per thousand pounds) that could be generated.

Over a period of time, a considerable amount of MEA will be lost out the top of the absorber as large volumes of gas carry entrained MEA out the top of the absorber in spite of preventive measures. Further MBA is lost due to pumping losses as large volumes of absorbent are required and therefore circulated to remove the great quantities of CO that are typically formed in modern hydrogen production plants. Other common CO absorption systems-for example, hot carbonateare generally similar to the alkanolamine system in the respects described above with only moderate reduction in regeneration heat requirements.

Since the alkanolamine absorbents tend to degrade, a reclaimer is commonly used to purify the absorbent. The reclaimer is essentially a small reboiler. It is fed a slip-stream of the absorbent from the bottom of the stripper. Only that portion of the slipstream that is vaporized is returned to the stripper system. Heavy tarry material collects in the bottom of the reclaimer and is periodically withdrawn and passed to sewerage as a spent alkanolamine stream. Common practice is to clean the reclaimer is frequently as once a week. The cleaning procedure typically involves taking the reclaimer off-stream, draining the spent alkanolamine and heavy tarry material, and steam cleaning the reclaimer.

It is thus apparent that cleaning the reclaimer will result in losses of absorbent in addition to those losses caused by entrainment and pumping leakage. Although the alkanolamine is expensive, this cleaning procedure is necessary to avoid build-up of corrosive bodies in the CO absorption system. Corrosion, which would be worse without the reclaimer, still is a considerable problem in the alkanolamine CO absorption systems.

Various absorption systems other than alkanolamine-CO absorption systems have been proposed in recent years. For example, U.S. Pat. No. 3,347,621 discloses a method of removing acidic gases including CO from gaseous mixtures by contacting the gaseous mixture with a liquid absorbent comprising diisopropanolamine and a cyclotetramethylene sulfone. The liquid absorbent may comprise l5-45 percent by weight of diisopropanolamine and55-85 percent by weight of the sulfone or 1-25 percent by weight of water, -45 percent by weight of diisopropanolamine and 50-84 percent by weight of the sulfone. The use of the above-mentioned absorbent is described further in the Oil and Gas Journal, June 30, l969, starting at page l 17. Preferably, the absorbent is composed of sulfolane (tetrahydrothiophene dioxide), DIPA (diisopropanolamine), and water. The relative amounts of these components can be varied to yield a composition which is suitable for the amounts of acid gases to be removed and other process variables.

The DlPA constituent accomplishes chemical absorption, while the relatively inert, sulfolane adds physical absorption ability to the solvent. This combination of absorption capabilities offers advantages both for loading and unloading of the solution.

The DlPA combines with sour components in an acid-base reaction essentially nonsensitive to pressure, whereas sulfolane adds physical solubility which is proportional to the acid gas pressure. The net result is a solvent having a good affinity for acid gas components at low-to-medium partial pressures, and an extremely high affinity for these components at high partial pressures.

Similar to the use of sulfolane and DIPA is the process disclosed in U.S. Pat. No. 3,352,63 l,'which is a process for separating carbon dioxide from gaseous mixtures by absorbing the carbon dioxide from the mixture in a liquid absorbent consisting of (a) 5-25 percent weight water, (b) l5-60 percent weight of monoethanolor diethanolamine or mixtures thereof, and (0) 28-80 percent weight ofa cyclotetramethylene sulfone (e.g., sulfolane).

Various physical absorbents have been disclosed. For example, U.S. Patent 2,649,166 discloses the use of dimethoxytetraethylene glycol as a physical absorbent for removing CO from gases such as hydrogen. U.S. Pat. No. 2,926,757 discloses the use of propylene carbonate as a physical absorbent for CO U.S. Pat. No. 3,l20,993 discloses physical absorbents such as N-alkylated lactams of the y or 5 amino acids derivable from butyric acid, valeric acid and caproic acid, namely, the N-alkylated pyrrolidones or piperidones which may be substituted with one or two lower alkyl groups, e.g., methyl groups, on the heterocyclic nucleus.

U.S. Pat. No. 3,225,519 discloses the use of a polar compound, specifically acetone, as a physical absorbent for CO U.S. Pat. No. 3,324,627 discloses the use of a polar solvent such as methanol together with a physical absorbent such as a pyrrolidone or a piperidone for CO absorption.

British Pat. No. l,024,4l2 discusses the use of combined physical and chemical absorbents in processes for the removal of acid gases from other gases such as refinery gases or light hydrocarbon gases. As indicated in the British Patent, the use of chemical absorbents such as alkanolamines involves the formation of alkanolamine salts of the acid gases, these salts being subsequently decomposed, usually by heating, to regenerate the solution and to drive off the absorbed compounds. Such solution, however, have only a relatively limited solubility for acid gases even when the latter are under a high partial pressure, which results in the necessity to use high solution-to-gas ratios.

British Pat. No. 1,024,4l2 points out that, as distinct from such chemical solvents" as the alkanolamines, certain classes of materials are regarded as physical solvents, such as N-alkylated pyrrolidones and N-alkylated piperidones. The latter solvents or absorbents appear to act in a purely physical manner, absorbing acidic gases physically without the formation of any apparent reaction product. However, one of the problems encountered with such physical solvents is the difficulty of obtaining a complete or substantially complete removal of the acidic contaminants.

According to the process disclosed in British Pat. No. 1,024,412, acidic gases are removed from gases such as light hydrocarbons by contacting the gas mixtures with a mixed solvent consisting of at least one weakly basic amine (such as mono-ethanol amine or diisopropanolamine) and a physical absorbent such as an N-alkylated pyrrolidone or an N-alkylated piperidone.

In the past, although physical absorbents and mix tures of chemical absorbents with physical absorbents have been used for removing acidic gases from natural gases and other pressurized hydrocarbon gas streams, mostly chemical absorbents have been used for removing CO from mixtures of CO with hydrogen when high purity hydrogen is desired as, for example, in a hydroconversion process such as hydrocracking or hydrotreating.

C. Compression of high purity hydrogen Some of the processes which use high purity hydrogen as a reactant are: hydrodesulfurization, operating at pressures between about 100 and 1500 psig; hydrotreating, operating at pressures between about 200 and 2000 psig; hydrocracking, operating at pressures between about 450 and 3000 psig; and thermal hydrodealkylation, operating at pressures between about 450 and 1000 psig. All of these just-mentioned hydroconversion processes may operate at even higher pressures (for example, up to 10,000 psig) than just given but seldom will operate at pressures lower than the range given. Thus, it can be seen that many of the processes which use hydrogen require the hydrogen at a high pressure, which in most cases means generated hydrogen gas must be compressed before being passed to a hydrogen-using process.

Basically, all compressors may be considered as belonging to one of two categories; i,e., their principles involve either that of true mechanical compression (positive displacement) or centrifugal compression. Compressors utilizing true mechanical compression are so considered because the act of volumetric reduction is accomplished by means of a compressing element. The compression element may be in the form of a piston which in its particular motion entraps and displaces gas within a suitably designed and fully enclosed housing. Motion may be reciprocating during which the element, in the form of a piston, passes back and forth within dimensional limits over the same course within a cylinder in a straight-line direction.

Centrifugal compression is accomplished by centrifugal force exerted on an entrapped gas during rotation of an impeller at high speed. Most centrifugal compressors depend primarily on centrifugal force and high tangential velocity of the fluidin the periphery of the impeller (or rotors or blades in the instance of some turbocompressors) to produce the desired head or discharge pressure. In this specification, the terms centrifugal compression" or compressor are meant to include turbine compression or turbocompressors, including, for example, axial-flow compressors. In the broad sense of centrifugal compression used herein, compression is effected, at least to a substantial degree, by conversion of velocity head to pressure head.

The reciprocating compression is used for hydrogen compression, but it has some severe disadvantages, particularly for large-size plants:

1. All parts are subject to unbalanced, reciprocating stresses; and foundations, frames and other parts must be large. To minimize vibration, speeds are low (400-700 RPM); and capacity is low. Therefore, in large plants, several machines are required. Cost of installing, instrumenting, protecting and piping several machines is high. Considerable land is required, and plants are bigger and more complex, making them more difficult to control.

2. The reciprocating machine is less reliable than centrifugal machines, and it is common practice to design plants with one or two expensive spare machines ready to come on-stream in the event of a failure.

3. The reciprocating machine produces a pulsating gas supply which sonically transmits vibration to piping instruments and other plant facilities. Such vibrations can cause hazardous failures with hydrogen at high pressure.

4. The low speed of reciprocating compressors tends to limit prime movers to low speed, electric motors or gas engines. While it is possible to use high speed steam or gas turbines, large reduction gears must be used. The pounding of the reciprocating loads has led to poor experience with these units. Hydrocracking and hydrogen manufacturing processes can be designed to produce byproduct steam if it could be used in steam turbine drivers. However, for the reasons just given, this byproduct steam is generally not used to drive the reciprocating compressors.

5. Reciprocating compressors are particularly susceptible to severe damage if liquid is present in the gas being compressed.

By comparison, centrifugal compressors are reliable, rugged, in most cases relatively simple, have large capacities, are relatively small, have balanced stresses, and generally cause relatively little vibration or pulsation in the plants. They can be driven by high speed, steam turbines or gas turbines.

However, centrifugal compressors cannot, with any reasonable degree of feasibility, be used as high purity hydrogen compressors.

Compression ratios (ratio of discharge pressure to inlet pressure for one stage of compression) obtainable with a centrifugal compressor are a function of the molecular weight of the gas to be compressed. With pure hydrogen having a molecular weight of 2, compression ratios are limited to about 1.025. Because of this low compression ratio for hydrogen, centrifugal compressors are not practical to date for compression of high purity hydrogen.

Table I below illustrates the sharp decrease in compression ratio for centrifugal compression as the molecular weight of the gas being compressed decreases. The number of stages used in the compression is the same for each case in Table I.

TABLE I Barometer, psia 14.4 14.4 14.4 Inlet temperature, F. 60.0 60.0 1 10.0 k (Cp/Cv for inlet gas) 1.11 1.398 1.36 Inlet capacity, cfm 20,000.0 20,000.0 20,000.0

Head, fL-lb. perlb. 22,000.0 22,000.0 22,000.0

Molecular weight 63.0 28.95 [0.1 Inlet pressure, psia l6.73 14.73 14.08 Discharge pressure, psia 79.53 29.73 I7.99 Compression ratio 4.75 2.01 1.28

As previously indicated, it is not practical to use centrifugal compressors to compress high purity hydrogen to high pressures because of the multitude of stages that would be required. For example, the centrifugal compression ratio (ratio of discharge pressure to inlet pressure for one stage of centrifugal compression) with hydrogen, molecular weight of 2, is limited to about 1.025. Consequently, over 75 stages of centrifugal compression would be necessary to bring the pressure of hydrogen up to 1,700 psig starting from a pressure of 200 psig. On the other hand, two stages ofa reciprocating positive displacement compressor could increase the pressure from 200 psig to 1,700 psig. Thus, in spite of their problems previously discussed, reciprocating compressors have heretofore been used in bringing high purity hydrogen to high pressure.

SUMMARY OF THE INVENTION According to the present invention, an improved process is provided for manufacturing high pressure, high purity hydrogen which comprises:

a. generating at a pressure below about 450 psig a hydrogen-rich gas containing sufficient CO so that the molecular weight of the hydrogen-rich gas is at least four;

b. centrifugally compressing the hydrogen-rich gas from the pressure below about 450 psig to a substantially higher pressure above 450 psig to obtain high pressure hydrogen-rich gas;

c. removing CO from the high pressure hydrogenrich gas to obtain high purity hydrogen, at least part of the CO being removed by absorbing CO in a mixed absorbent comprising a chemical absorbent and a physical absorbent.

The amount of chemical absorbent mixed with the physical absorbent to comprise the mixed absorbent is not a critical aspect of the present invention, although it is preferred to use minor amounts of chemical absorbent and major amounts of physical absorbent. The amount of chemical absorbent in the mixed absorbent can be between about 1.0 and 49 weight percent chemical absorbent based on the total weight of the absorbent mixture, but preferably is between about 5.0 and weight percent. The amount of physical absorbent in the mixed absorbent mixture can be between 51 and 99 weight percent, but preferably is between about 75 and 90 weight percent. Other components besides the chemical absorbent and physical absorbent can be present in the mixed absorbent. Thus, the mixed absorbent can also contain water.

Heretofore, centrifugal compression has not been used to bring high purity hydrogen to high pressures used, for example, in hydrocracking. This is because of the inapplicability of centrifugal compressors in compressing high purity hydrogen to high pressures, generally above 450 psig, and most frequently above 900 psig.

The present invention is based on the idea that the raw hydrogen may advantageously be compressed before CO removal, thus making possible the use of a centrifugal compressor because of the higher molecular weight of the gas mixture of H and CO It is surprisingly found that, in spite of the facts that much energy is expended in bringing the CO to a high pressure and what is desired is to bring only the high purity hydrogen to a high pressure since the CO will ordinarily be vented, the new method provides an overall more advantageous process.

According to an alternate embodiment of the present invention, purified H gas is obtained by centrifugally compressing a raw, H -rich gas containing CO to high pressure and then removing the CO at high pressure by cooling to condense out a portion of the CO and removing residual (left after removing the condensed liquid CO gaseous CO from the H gas using a mixed absorbent comprising a chemical absorbent and a physical absorbent to physically absorb the residual CO According to a preferred embodiment of the present invention as integrated with a hydroconversion process, the purified H gas is reacted with hydrocarbons in a hydroconversion zone; and the hot effluent from the hydroconversion zone is used to generate at least part of the steam to drive the centrifugal compressors which are used to compress the raw, H -rich gas containing CO Some of the more salient advantages realized in accordance with the present invention and preferred embodiments of the present invention are:

1. centrifugal compression replaces reciprocating compression and substantially eliminates problems arising from reciprocating compression in hydrogen production. Simpler, less expensive, more reliable plants result. Maintenance expenses, including labor and parts required to repair the positive displacement reciprocating compressors, are reduced. Also, the possibility and/or amount of production stoppage due to compressor breakdown is greatly reduced. Hazard due to equipment failure on high pressure equipment is reduced.

2. Because of the high pressure obtained by centrifugally compressing the H and CO nearly all of the CO may be removed from the H relatively easily by physical absorption of the C0 The chemical absorbent which is used together with the physical absorbent operates to improve the ability of the absorption system to reduce the CO concentration in the purified hydrogen gas down to very low levels, for example, below about 1,000 parts per million by volume of CO in the purified hydrogen gas.

In using the physical absorption, the solubility with respect to temperature and pressure follows the normal trends; that is, increased solubility with decrease in temperature or increase in pressure. Thus, the highpressure works to the advantage of the absorption step in two respects. It increases the ability of the absorbent to absorb CO because of the high pressures. Also, since the high pressures result in a system that is easily completely or nearly completely self-refrigerated, low temperatures are economically obtainable so that the absorption may be carried out at low temperature where the solubility of CO (or CO and H 8) in the absorbent is relatively high. The refrigerating of the absorption step is effected primarily by expansion of released CO which is desorbed from the high pressure, CO -rich absorbent by reducing the pressure on the CO -rich absorbent.

The high pressure required in the absorption step of the present invention generally is between about 450 psig and 4000 psig, but pressures as high as 5000 to 10,000 psig may be employed. However, the more usual pressure range is between 900 psig and 3000 psig.

3. Because of the improved methods of removing CO less heat is needed in the CO removal step. Specifically, less heat is needed to regenerate the CO absorbent. This is largely because the absorbent is regenerated primarily or, in some instances, solely by reducing the pressure on the absorbent so as to release the CO which has been physically absorbed (except for the minor small amounts of CO which are mostly lastly absorbed in the absorption step by the chemical absorbent) at the high pressure obtained by centrifugal compression of the H -rich gas containing C 4. Since less heat is needed in the CO removal step, which heat is generally supplied by the effluent from the steam reforming or partial oxidation zone, this effluent heat is now used, for example, to generate steam which is readily useable in the high speed turbines driving the high speed, raw hydrogen gas centrifugal compressor or compressors.

5. Because the CO is brought to a high pressure as well as the hydrogen, improved methods of removing CO fit advantageously into the hydrogen production scheme. For example, condensation of CO is attainable much more readily at the higher pressures because extremely low temperatures are not required. Initial condensation of CO is obtained by cooling to about 0F. if the partial pressure of CO is at least 306 psia (306 psia corresponds to about percent CO in a H CO gas mixture at 1500 psia). Further condensation is attainable by cooling to lower temperatures with the result that only a fraction of the original CO is left in the hydrogen-rich gas. Thus, in a preferred embodiment of the present invention, the CO absorption requirements of the absorbent are reduced as the absorbent only is required to absorb the fractional amount of CO left in the hydrogen-rich gas after condensation and separation of a large (e.g., from to 95 percent, preferably about 50 to 70 percent) portion of the CO in the raw, hydrogen gas mixture.

Since the absorption takes place at high pressures and since at least part of the CO is advantageously condensed at high pressures, much of the cooling required for the low temperature absorption can be obtained by subsequent reductions of pressure on the condensed CO and/or the CO -rich absorbent. The absorbent is regenerated primarily by expansion to a lower pressure. The desorption of the carbon dioxide effects substantial cooling of the absorbent and thus lowers the temperature of the absorbent which was earlier raised by the heat of absorbing CO Also the high pressure condensed CO separated from the raw hydrogen gas can be used to remove large quantities of heat from the feed to the CO absorber, as the condensed CO is expanded to substantially lower pressures and is therefore available as a refrigerant.

6. Heat available from the effluent of a hydroconversion zone using the high purity, high pressure hydrogen from the hydrogen production zone can be used to generate steam to drive the centrifugal compressor of the hydrogen production zone.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic flow sheet of a preferred embodiment of the invented process. The major process steps shown in the drawing include: facilities for generating H -rieh gas containing CO steam boilers for producing steam to drive the centrifugal compressor, a centrifugal compressor for obtaining high pressure H -rich gas containing CO heat exchange for cooling the H -rich gas containing CO and condensing a part of the CO absorption facilities for removing CO (or CO and H 5) from the H- -rich gas, and a hydroconversion zone for reacting the purified H gas with hydrocarbons in reactors, with the hot effluent from these hydroconversion reactors being used to generate steam to drive the centrifugal compressor.

DETAILED DESCRIPTION AND DESCRIPTION OF THE DRAWING Referring now in more detail to the embodiment of the invention shown in the drawing, partial oxidation or steam reforming is carried out in accordance with wellknown processes in zone 12.

In the case of partial oxidation, a hydrocarbon, which may be a gas, oil or a solid, is fed to the partial oxidation zone through line 10. Steam and/or oxygen is fed to the partial oxidation zone through line 11. The hydrocarbon and oxygen are reacted in a partial oxidation reactor at elevated temperatures, either with or without catalyst, according to well-known processes to produce H CO and CO Pressure in the partial oxidation reactor typically is between 200 and 600 psig and temperature between 2000 and 3200F.

H 5 is formed in the partial oxidation reaction if the hydrocarbon feed to the partial oxidation reactor contains sulfur compounds. The H 8 formed may be removed before passing the partial oxidation reactor effluent to the CO shift conversion reactor. If the partial oxidation reaction is conducted at high pressuree.g., in excess of 600 psigthe H 8 may be removed using a physical absorbent or a combination physical absorbent with a chemical absorbent. In the present invention, it is advantageous to remove the H 5 with the concurrent removal of CO prior to the CO shift conversion step, as there is sufficient CO formed in the CO shift conversion step to raise the molecular weight of the hydrogen-rich gas stream to at least four, thus making feasible the use of centrifugal compression for obtaining a high pressure H CO gas mixture.

Because there are, however, sulfur resistant CO shift conversion catalysts available, the H 8 may alternatively be left in the hydrogen-rich gas feed to the CO shift converter. In this case, it is preferred to compress the H -CO -H S gas mixture from the CO shift converter to high pressure and then remove the CO and H 8 by absorption in a mixed absorbent comprising a chemical absorbent and a physical absorbent, or a combination of condensation and absorption in a mixed absorbent comprising a physical absorbent and a chemical absorbent of the CO and H 8.

In the case of steam reforming, a desulfurized feedstock is mixed with superheated steam and charged to a steam reforming furnace. The furnace consists of a combustion chamber in which are located alloy tubes containing a nickel base catalyst. While passing over the catalyst, the hydrocarbons are converted to H CO and CO. The furnace supplies the heat needed to maintain the temperature for the endothermic reaction. Outlet temperature from the reforming furnace is usually maintained between l4001620F., and pressure maintained between about 75 and 450 psig. As in the case of partial oxidation, carbon monoxide in the effluent hydrogen-rich gas from the steam reformer is reacted with water in one or more shift reactors to produce more H and CO and reduce the CO content to about 0.22 volume percent.

The raw hydrogen gas from zone 12 comprising predominantly H and containing a minor amount of CO (and, in some cases, also containing H S) passes via line 13 from the shift conversion step in zone 12 to a steam boiler El where steam is generated from the boiler feed water passed through lines 60 and 61 into E-l. The raw hydrogen gas which has been cooled in steam boiler E-l passes through water cooler 14 and then to separator 15. Water that has been condensed from the raw hydrogen gas is removed through line 70 and passed to further processing now shown. The raw hydrogen gas is withdrawn from the separator through line 16 and passed to centrifugal compressor 17. The centrifugal compressor substantially increases the pressure of the H CO CO gas mixture; i.e., the pressure is increased by at least 200 psi. Preferably, the pressure is increased by 450 psi, and still more preferably, the pressure is increased by as much as 1000 or 1500 psi. For example, when the raw hydrogen gas is generated by steam reforming, the pressure is advantageously raised from below about 450 psig to above about 900 psig, preferably above 1300 psig. Presence of the CO in the raw, hydrogen-rich gas that is passed to the centrifugal compressor not only increases the molecular weight of the gas mixture but also increases the volume of the gas mixture. Both of these factors increase the applicability of centrifugal compressors as opposed to reciprocating.

In the preferred embodiment shown in the drawing, all of the gas from H manufacturing zone 12 is passed to the centrifugal compressor. Because of the 17 to 40 volume percent CO present, along with other impurities such as CO, the molecular weight of this H -rich gas is between about and 25. Within the concept of the present invention, it is practical to lower the molecular weight of the H -rich gas to as low as about 4 by removing part of the CO and then using centrifugal compressors for compression to high pressure. Thus, an alternate to the embodiment of the present invention wherein the hydrogen-rich gas is compressed before CO removal is centrifugal compression after partial CO removal but before final CO removal. A gas mixture of H and CO containing about 5 volume percent CO has a molecular weight of four.

High pressure, raw hydrogen gas is withdrawn from the centrifugal compressor through line 18 and cooled by heat exchange with cooling water in exchanger E-3. To avoid icing in the embodiment illustrated in the drawing, antifreeze is added through line 19 to the effluent raw hydrogen gas from E-3. Acetone or methanol, for example, may be used as the antifreeze. Then the gas is passed in line 21 through exchanger E4 and further cooled by a refrigerant, for example,

ammonia. The refrigerant will serve to remove imbalance heat loads from the system during start-up and also to remove excess heat that is not removed by the CO and absorbent that are expanded after the CO absorption, as is described below. For purposes of illustration, methanol is used as the preferred absorbent in the description of the drawing, although other physical absorbents may be advantageously used.

As discussed in the prior art references mentioned previously, certain classes of materials are regarded as physical absorbents whereas other materials are chemical absorbents. Examples of physical absorbents are methanol, and other monovalent aliphatic alcohols, such as ethyl alcohol and propyl alcohol, or cyclic alcohols, such as cyclopentanol and cyclohexanol; aliphatic polyvalent alcohols, such as ethylene glycol, propylene glycol and glycerin; lower ketones, such as acetone, diethylketone, methylbutylketone; N-alkylated pyrrolidones, such as N-methyl pyrrolidone; N-alkylated piperidones; and sulfones, such as tetrahydrothiophene 1,1-dioxide and the homologues thereof; dialkyl ethers of a polyalkylene glycol such as the dimethyl ether of diethylene glycol, the dimethyl ether of triethylene glycol, the dimethyl ether of tetraethylene glycol, the dimethyl ether of pentaethylene glycol, the dimethyl ether of hexaethylene glycol, and the dimethyl ether of heptaethylene glycol. Particularly preferred physical absorbents for use in the present invention are propylene carbonate, acetone, and methanol.

As is indicated by our previous application Ser. No. 736,520, the use of a physical absorbent by itself as, for example, propylene carbonate together with centrifugal compression of the H -CO gas mixture to high pressures before removing the CO by physical absorption, results in an unexpectedly attractive and advantageous process for the production of high pressure high purity hydrogen. The present patent application is directed more narrowly to the use of not only a physical absorbent but an absorbent system comprising a chemical absorbent together with a physical absorbent for removing CO from H CO gas mixtures which have been centrifugally compressed to high pressure. Thus, according to the present invention, although CO is absorbed primarily by high pressure physical absorption, a minor amount (no more than 49 weight percent) of chemical absorbent can be mixed with a physical absorbent to increase the flexibility of the CO absorption step and to increase the effectiveness or efficiency of the absorption step in lowering the CO content in the hydrogen gas to very low concentrations. Thus, according to the present invention, it has been determined that it is advantageous in combination with centrifugal compression of H CO gas mixtures to reduce the CO concentration to about 500 parts per million by volume or lower by mixing with a physical absorbent a chemical absorbent such as alkanolamines or other compounds whose absorption mechanism involves the formation of salts or other decomposable reaction products; i.e., products which when heated decompose to release the chemically absorbed CO and thus regenerate the chemical absorbent. Among the alkanolamines which act as chemical absorbents, monoethanolamine and diethanolamine can be used with particular advantage. Other typical chemical absorbents are dipropanolamine, diisopropanolamine, alkylamines, phenyl alkyl amines, alkoxy alkyl amines and alkoxy aryl amines. In contrast to physical absorbents, chemical absorbents, such as MEQ, have little increased ability to absorb CO at high pressures versus lower pressures of about 250 psig, but the chemical absorbents do generally have the property of being relatively more effective for CO removal at relatively low CO partial pressures (which, of course, exist even in a high pressure H CO gas mixture when the CO concentration has been reduced to a relatively small amount).

The raw hydrogen gas passes in line 22 to exchanger E5 where it is further cooled by expanding CO that was previously condensed by cooling the high pressure raw hydrogen gas. Thus, expanding low temperature CO is used to cool high pressure raw hydrogen gas and condense C0 The raw hydrogen gas in line 23 from -5 then passes through E-6 where it is still further cooled (with resultant increased CO condensation) by exchange with the absorber overhead. The effluent from 15-6, comprising H liquid CO or CO and H 8, and residual CO or CO and H 8, at a pressure of, for example, 1700 psig, is passed in line 24 to absorber 25. In absorber 25, CO or CO: and H 8, that has been condensed due to cooling in the preceding exchangers, drops to the bottom; and residual, gaseous CO or CO and H 8 is absorbed in methanol absorbent, preferably mixed with a minor amount of chemical absorbent, and thus removed from the raw hydrogen gas as the gases pass upward in absorber 25. Hydrogen gas containing about 1.0 percent CO (and generally less than 1 percent when a chemical absorbent is used together with the methanol absorbent) and 1.0 to 2.5 percent CO by volume passes out the absorber in line 48. After transferring heat in exchanger 13-6, the hydrogen gas stream is passed in line 49 to methanation zone 50.

Rich absorbent, laden with CO and possibly H 8, is removed along with condensed liquid CO in line 26 from the bottom of absorber 25. After reducing the pressure, the absorbent and liquid CO are passed in line 27 to first letdown drum 28 where a small amount of dissolved hydrogen is flashed off and removed in line 29. This hydrogen is recycled to the compressor for recovery via line 71. Because the hydrogen is much more volatile and less soluble in the absorbent than CO the hydrogen may be removed as a gas while the CO remains in the liquid phase. The absorbent and liquid CO are passed from the first letdown drum in line 30 to expander 31 where the pressure is further reduced. The expander 31 recovers work energy from the expanding and vaporizing CO which work energy can be used, for example, to drive absorbent pump 36. CO and absorbent leave expander 31 in line 32 to second letdown drum 33. Low temperature, gaseous CO resulting from the expansion is withdrawn from the second letdown drum in line 34 and passed through E-S countercurrent to high pressure H -rich gas containing CO to remove heat from the H -rich gas containing CO In the partial oxidation process, the CO in line 34 may contain H 8; and it may be advantageous to process this gas for H S removal from the CO before warming in E-5. This can be done in a separate column with a small stream of purified absorbent from line 47.

CO is withdrawn from heat exchanger 13-5 in line 9 and vented or passed to further processing.

Methanol absorbent, together with a minor amount of chemical absorbent, in line 35 from the bottom of the second letdown drum contains a small amount of CO not flashed off as a gas in the second letdown drum. This absorbent, which has been cooled to about F. due to CO flashing off from the absorbent, is passed in part to an intermediate point in absorber 25 via lines 37 and 38. Although the absorbent in line 38 has not been completely freed of CO it is still effective to remove CO because of its low temperature (which tends to condense C0 0, as it is introduced to the absorber at a point where the CO concentration is sufficiently high that there is ample driving force from the gas phase into the liquid CO absorbent phase. The remaining part of the absorbent is passed in line 39 through cooler E-7 into the top of absorbent regenerator 41 via line 40. The reboiled regenerator 41 frees this part of the absorbent of essentially all CO or CO and H S remaining in the absorbent removed from the second letdown drum. The thus purified absorbent is withdrawn from absorbent regenerator 41 in line 46 at an intermediate position, below absorbent inlet line 40 but substantially above the bottom of regenerator 41. This lean absorbent is cooled to about 65F. by passing through E-7 countercurrent to the regenerator feed. The lean absorbent is withdrawn from E-7 in line 47 and passed to the top part of absorber 25.

Reboiling heat is supplied to the regenerator 41 by reboiler 13-13, which reboils a portion of the regenerator bottoms using steam as a heating medium. A small amount of water is withdrawn from the bottom of regenerator 41 in line 43. CO is withdrawn in line 42 from the top of regenerator 41 as a gas and is vented or passed to further processing. In the case where partial oxidation is used to generate the raw l-I -rich gas, typically the gas withdrawn in line 42 from regenerator 41 will contain H 8. In this instance, the gases from the regenerator may be passed to a Claus process for the production of sulfur if there is sufficient H 8 in the CO --H S gas mixture in line 42.

In methanation zone 50, the CO and CO in the hydrogen-rich gas effluent from absorber 25 are converted to methane and steam by the following reactions:

High pressure, in excess of 450 psig, favors the conversion of carbon oxides in the methanation zone. High purity hydrogen, containing no more than 20 parts per million carbon oxides and about 4 percent other impurities, such as CH is withdrawn in line 51 from the methanation zone. It is passed together with recycle hydrogen through lines 53 and 54 and reacted with hydrocarbons in the hydroconversion zone 55.

Alternatively to the methanation zone, a copper liquor system may be employed to remove CO, as the copper liquor systems are advantageously used at high pressure for absorbing CO so as to obtain high purity hydrogen. In general, the term high purity hydrogen" connotes hydrogen purity of about 96 to 99+ volume percent, with the balance being impurities comprising CH N and small amounts of carbon oxides. But in some cases, the high purity hydrogen may contain 5 to 8 percent impurities by volume, particularly when the CH content is relatively high in the product hydrogen. The term high purity hydrogen" is used to refer to the hydrogen either before or after methanation or alternative processing for reduction of the carbon oxides from small concentrations down to the parts per million level.

Hydrocarbon feed enters the hydroconversion zone through lines 52 and 54. The hydroconversion zone may, for example, be a hydrodesulfurization unit operated at about 1,700 psig and 700F. reactor pressure and temperature, respectively, and consuming about 500 SCF of hydrogen per barrel of hydrocarbon feed to the hydrodesulfurization unit. As indicated previously, other hydroconversion processes include hydrocracking, hydrotreating or hydrofining and thermal hydrodealkylation. The effluent from the hydroconversion zone, at temperatures between 400-1,200F., generally between 400800F. is withdrawn in line 56. This high temperature effluent stream is passed through steam boiler E2 wherein heat is removed from the hydroconversion zone effluent to generate steam. The steam is generated from the boiler feed water passed through lines 60 and 62 into the steam boiler. Steam is withdrawn from steam boiler E-l in line 64 and from steam boiler E-2 in line 63 and passed in line 65 to turbine driver 67. Thus steam is produced through heat recovery from the hydroconversion zone as well as from the hydrogen production zone to drive the turbine driver 67. The turbine driver 67 in turn furnishes the motive power for raw hydrogen gas centrifugal compressor 17. Either back pressure steam turbines or condensing steam turbines are used to drive compressor 17. Exhaust steam is withdrawn from the turbine in line 66.

After heat is recovered from the hydroconversion zone effluent in steam boiler E-2, the effluent is passed in line 57 to separator 58. Recycle hydrogen is withdrawn from the separator in line 68; and oil is withdrawn from the bottom of the separator in line 59 for further processing, such as fractionation or further hydroconversion.

Steam generated by the steam boilers in addition to that required by turbine driver 67 is withdrawn in line 69 and is used to reboil the absorbent regenerator reboiler E-8 and to supply steam to partial oxidation or steam reforming zone 12. In the case of steam reforming, a large quantity of steam is generated by removing heat from the reforming reactor effluent prior to CO shift conversion. This steam may be used to drive turbine driver 67 and to supply steam needed for the steam reforming reaction. Exhaust steam from turbine driver 67 is used elsewhere or is condensed depending upon the design of the turbine driver.

EXAMPLE 1 The following example is based on calculated improvements (which could be made using the principles of this invention) to a new plant which produces 135 MMSCFD (million standard cubic feet per day) hydrogen.

Natural gas is fed in line 10 and steam in line 11 to a steam methane reformer furnace. Furnace effluent comprising H CO and CO is quenched by spraying water into the hot gas; and then the effluent is successively passed through a high temperature CO shift converter, a second water spray quencher, and a low temperature CO shift converter to convert water and CO to CO plus hydrogen. A wet, gaseous stream at about 500F. and containing about percent hydrogen and 18 percent CO (dry basis) is then cooled in steam boiler E-l, first generating 150 psig steam and then 40 psig, and then heating boiler feed water. it is finally cooled with cooling water to condense most of the excess steam in the process gas. A total of about 340,000 lb./hr. of steam is recovered. When augmented with 20,000 lb./hr of steam generated in steam boiler E-2 and used in turbine driver 67 (which exhausts to watercooled condensers operating under vacuum), about 24,000 BHP is produced. This is enough to drive centrifugal compressor 17 compressing the hydrogen and CO from a pressure of 225 psig to l700 psig. Because of the reliability and large size of the centrifugal compressor, only one multistage unit is required.

CO is then removed from the high pressure gas in absorber 25 using semilean absorbent (methanol together with a minor amount of chemical absorbent such as N-methyl 2-pyrrolidone) at 90F. in line 38 and lean absorbent at F. in line 47. Semilean absorbent is absorbent which has not been completely regenerated and contains more CO than the essentially completely regenerated absorbent withdrawn in line 46 from absorbent regenerator 41. At 1700 psig pressure, the absorption system is essentially autorefrigerated by expansion of the high pressure CO As explained under Detailed Description," the expanded CO is used to cool the H -CO gas feed to absorber 25. Also, the absorbent is cooled by the CO as CO flashes off from the absorbent as pressure is reduced in expanding the Co -rich absorbent across expander 31. Preferably, any small amounts of CO and CO left in the gas from absorber 25 is converted to methane in methanator zone 50. High purity hydrogen is then fed at high pressure to hydroconversion zone 55 for upgrading hydrocarbons introduced in line 52.

In contrast, according to prior art, CO is removed from the H CO gaseous effluent from the CO shift converter using monoethanolamine (MEA) absorbent in an absorber operating at 250 psig. Instead of using the heat in the H -rich gas prior to CO removal to generate 340,000 lb./hr. of steam to drive the compressors (as in the invention) in accordance with the prior art, the heat is used to reboil (regenerate) the CO -rich MEA. To compress the high purity hydrogen according to the prior art, four reciprocating compressors are required (one spare). The three operating reciprocating compressors require 19,000 BHP to compress the high purity hydrogen from 225 psig to 1700 psig. Cost of electricity consumed by the electric motors used to drive the reciprocating compressors is $850,000 per year (19,000 HP X 0.746 KWHr/HP X $7.22/l,000 KWHr X 365 X 24 X 0.95 Op. Factor).

In the present invention, heat is recovered from the hydroconversion zone 55 effluent to produce, in accordance with this example, about 20,000 pounds per hour of steam. This 20,000 pounds per hour is worth about $65,000/year. Making the conservative assumption that this 20,000 pounds per hour of steam could be utilized elsewhere, and also taking into account about $75,000 per year as the cost of steam for the absorbent regenerator of the present invention and for a small steam-driven refrigeration compressor used in the present invention, the net annual savings using the process of the present invention is $710,000 ($850,000 $65,000 $75,000) for a hydrogen plant producing 135 million standard cubic feet per day of hydrogen.

In addition to this $710,000 annual savings is the important reduction in plant complexity and improved maintenance and safety from the use of centrifugal compressors instead of reciprocating compressors used in the hydrogen production process of the prior art.

EXAMPLE 2 This example illustrates a preferred embodiment of the present invention wherein none of the CO is removed by condensation. More particularly, the present example summarizes estimated investment and operating costs calculated for a process in accordance with the prior art using monoethanolamine (MEA) for CO removal vs. a process in accordance with a preferred embodiment of the present invention using propylene carbonate (preferably with a minor amount of chemical absorbent to aid in reducing the CO in the purified hydrogen gas to very low amounts) for CO removal in a 75 MMSCFD per day steam methane reforming hydrogen plant. The MBA absorbent is used in a conventional chemical absorption system, which is operated at relatively low pressures. The propylene carbonate absorbent is used in a high pressure physical absorption system which can use a variety of solvents, such as acetone or n-methylpyrrolidone.

In either process, the effluent from the shift converter has the following composition in moles per hour: H 8,849; CO 2,253; CO: 28; N 51; CH,,: 227; H 13; for a total of 1 1,421 moles per hour. The molecular weight of this gas mixture effluent from the CO shift converter is 10.8. The amount of the gas stream in pounds per hour is thus about 123,000. The gas leaves the shift converter at a pressure of 250 psig.

In the process according to the prior art using MEA absorbent for removal of CO this gas mixture at 250 psig is passed to a C0 absorber wherein MEA removes the CO from the hydrogen-rich gas mixture by chemical absorption of the CO High purity hydrogen obtained from the top of the CO absorber is compressed by reciprocating compressors to 1,085 psig so that the pressure of the high purity hydrogen is high enough for use in a hydrocracking unit. The MEA absorbent is regenerated; that is, CO is removed from the MEA by stripping in a C0 stripper. The CO stripper uses a steam reboiler to generate hot stripping vapors in the bottom of the CO stripper and decompose the MEA- CO chemical bond at a temperature of about 200F. The stripped MBA is removed from the bottom of the stripper, cooled to about 100F., and passed to the upper part of the CO absorber.

For application of the propylene carbonate removal process in accordance with a preferred embodiment of the present invention, the hydrogen-rich gas effluent from the CO shift converter having a molecular weight of 10.8 is centrifugally compressed to a pressure of 1,1 psig. The temperature of the compressed hydrogen-rich gas is 1 10F. This high pressure feed gas is partially dehydrated by glycol injection before CO removal both to avoid hydrates and to maintain the solvent in a nearly anhydrous state.

In order to take advantage of the physical absorbents higher CO capacity at lowered temperatures and to minimize absorbent vaporization losses, the absorption operation is caused to take place at reduced temperatures. This is accomplished by the use of a small propane refrigeration system. Thus the high pressure hydrogen-rich feed gas is cooled to about 25F. and then fed to the CO absorber wherein cold propylene carbonate absorbs C0 The propylene carbonate absorbent is both regenerated and cooled to about 0F. by reducing the pressure on the CO -rich absorbent withdrawn from the bottom of the CO absorber. Purified hydrogen exits from the top of the CO absorber at about 12F. The purified hydrogen passes in counter-current heat exchange with the centrifugal compressor hydrogen-rich gas effluent so that the purifled hydrogen gas stream is heated to F. The pressure of the purified hydrogen-rich gas stream after this heat exchange is about 1,085 psig. The composition in moles per hour is: H 8,689; C0 1 CO: 28; N 5 1; CH 227; for a total of 9,085 moles per hour. The molecular weight of this purified hydrogen stream is about 3.01. So that the purified hydrogen will be more suitable for use in a hydrocracker, the purified hydrogen stream is passed to a methanator for conversion of CO and CO to CH Table 11 below shows that the propylene carbonate system operating at 1,100 psig has an estimated savings of $1.96 million in investment and 4-year operating costs when compared with an optimistic MEA system. When compared with an MBA system which has a C0 pickup of 0.4 mole CO per mole MEA (which is more in line with actual operating experience), the savings are about $2.63 million. This estimate is indicative of the savings that can be realized from a process which first compresses the shift converter effluent gases and then removes the CO by high pressure physical absorption.

The use of centrifugal compressors contributes a major portion of the savings obtained according to the present invention. For the estimates in Table 11, the propylene carbonate process is based on the centrifugal compressor operating at 68 percent efficiency to compress the CO -Containing gas from about 250 psig to 1,1 10 psig. An investment cost of $260 per brake horsepower was used for the reciprocating compressor used to compress purified hydrogen obtained after CO removal using MEA absorbent.

The hydrogen stream obtained after physically absorbing the CO in a propylene carbonate absorbent contains about 1 percent CO impurity. After methanation, the hydrogen purity is thus about 1 percent less than for an MBA system which removes essentially all of the C0 Table 11 includes an estimated $50,000 incremental investment required in the hydrocracking unit for a 1 percent decrease in purity in the purified hydrogen which is used in the hydrocracker. Product hydrogen purity obtained in a process using steam methane reforming with propylene carbonate for CO removal is estimated to be to 96 percent, depending on the nitrogen in the feed gas to the reformer.

When using the preferred absorbent system according to the process of the present invention, i.e., an absorbent mixture comprising a chemical absorbent and a physical absorbent, the CO can be removed from the centrifugally compressed H CO gas mixture down to levels considerably less than 1 percent CO impurity. Using the preferred mixed absorbent in accordance with the process of the present invention, the CO can be reduced down to about 500 ppm or less and frequently can be reduced to as low as 20-200-200 ppm CO by volume in the purified hydrogen gas. When the CO is reduced down to these low levels, the amount of methanation required to convert CO to CH is considerably reduced and in many instances, the methanation step can be wholly omitted, as in those cases wherein the hydroconversion process using the high pressure high purity hydrogen gas can tolerate about to 200 ppm CO in the hydrogen.

TABLE II CO REMOVAL AND H COMPRESSION FACILITIES 75 MSCFD HYDROGEN PLANT COST ESTIMATES IN THOUSANDS OF DOLLARS Based on wt. MEA aqueous solution and CC) pickup of 0.4 mole CO,/mole MEA.

Based on 20 wt. MEA aqueous solution and CO, pickup of 0.5 mole Co /mole MEA.

The propylene carbonate process requires an increased hydrocracker investment because hydrogen purity decreases by ab; ut I%. This CO, impurity could be reduced if a mixture of propylene carbonate with a chemical absorbent is used as the absorbent.

It is to be understood that the forms of the invention shown and described herein are to be taken only as preferred embodiments. Various changes may be made while still remaining within the spirit and essence of the invention, such as in the arrangement of heat exchangers shown in the drawing, particularly those used to cool the effluent from the centrifugal compressor; the methanation step may be omitted where it is found preferable to produce H containing residual amounts of CO and CO rather than CH the CO may be removed in two or more absorbers with, for example, centrifugal compression and/or low temperatures CO shift conversion between CO absorbers; excess steam may be withdrawn from the steam system at various points other than indicated; and the steam may be used for additional functions, such as driving the absorbent pump instead of using the expander for this service. The invention may advantageously be applied in different embodiments than shown where it is desirable to obtain high pressure, high purity hydrogen. Accordingly, the invention is not to be construed as limited to the specific embodiments illustrated but only as defined in the appended claims.

What is claimed is:

l. A process for manufacturing high pressure, high purity hydrogen which comprises:

a. generating at a pressure below about 450 psig a hydrogen-rich gas containing sufficient CO so that the molecular weight of the hydrogen-rich gas is at least four;

. centrifugally compressing the hydrogen-rich gas from a pressure below about 450 psig to a substantially higher pressure above 450 psig to obtain high pressure hydrogen-rich gas;

. removing CO from the high pressure hydrogenrich gas to obtain high purity hydrogen, at least part of the CO being removed by absorbing CO in a mixed absorbent comprising a chemical absorbent and a physical absorbent.

2. A process in accordance with claim 1 wherein the chemical absorbent is an alkanolamine.

- 3. A process in accordance with claim 1 wherein the chemical absorbent comprises a material selected from the group consisting of monoethanolamine, dipropanolamine and diisopropanolamine, and the physical absorbent comprises a material selected from the group consisting of methanol, acetone, propylene carbonate N-methyl pyrrolidone, tetrahydrothiophene l,l-dioxide and a dimethyl ether of tetraethylene glycol.

4. A process in accordance with claim 1 wherein the chemical absorbent comprises an alkanolamine and the physical absorbent comprises tetrahydrothiophene 1,1- dioxide.

5. A process in accordance with claim 1 wherein the hydrogen-rich gas containing CO is generated by steam reforming of hydrocarbons.

6. A process in accordance with claim 1 wherein the hydrogen-rich gas containing CO is generated by partial oxidation of hydrocarbons.

7. A process in accordance with claim 1 wherein the hydrogen-rich gas at a pressure below about 450 psig is centrifugally compressed to a pressure of at least 900 psig.

8. In a process for manufacturing high purity hydrogen by a train of steps comprising reacting hydrocarbons with oxygen-affording gas at conditions including a pressure below about 900 psig and elevated temperature to form a gas mixture comprising H CO and CO reacting CO in the mixture with H O to form additional H and CO and removing the CO to provide high purity hydrogen, the improvement for providing the high purity hydrogen at elevated pressure which comprises centrifugally compressing the gas mixture comprising H and CO from the pressure below about 900 psig to a substantially higher pressure above 900 psig, then removing CO from the compressed gas by absorbing CO in a mixed absorbent comprising a chemical absorbent and a physical absorbent at a pressure above 900 psig to thus obtain high purity hydrogen.

9. A process for producing high pressure hydrogen for use in a hydrocarbon hydroconversion zone which comprises the following steps:

a. steam reforming the light hydrocarbons at a pressure between one atmosphere and 450 psig to obtain a gas mixture comprised of hydrogen, CO and C02;

b. passing the gas mixture comprising hydrogen, CO and CO to at least one shift converter wherein the CO is converted to H and CO and an effluent is obtained comprised of H and CO c. compressing the effluent from the shift converter comprising H and CO in a centrifugal compressor to a pressure of at least 900 psig to obtain a high pressure gas mixture comprised of H and CO2;

d. removing CO from the high pressure gas mixture comprised of H and CO to obtain high purity hydrogen, at least part of the CO being removed by absorbing CO at high pressure in a mixed absorbent comprising a chemical absorbent and a physical absorbent;

e. passing the high purity hydrogen to a hydroconversion zone wherein hydrogen is reacted with hydrocarbons and a hot hydroconversion zone effluent is obtained;

f. passing said hot hydroconversion zone effluent through a steam boiler to generate steam;

g. driving said centrifugal compressor using: (1) said steam generated by heat obtained from said hot hydroconversion zone effluent; plus (2) steam generated by heat obtained from said shift converter effluent.

10. A process for producing high pressure hydrogen for use in a hydrocarbon hydroconversion zone which comprises the following steps:

a. steam reforming of light hydrocarbons at a pressure between one atmosphere and 450 psig to obtain a gas mixture comprising hydrogen, CO and CO2; I

b. passing the gas mixture comprising hydrogen, CO and CO to at least one shift converter wherein the CO is converted to H and CO and an effluent is obtained comprising H and C 0. compressing the effluent from the shift converter comprising H and CO in a centrifugal compressor to a pressure of at least 900 psig to obtain a high pressure gas mixture comprising H and CO d. cooling the high pressure gas mixture comprised of H and CO to a sufficiently low temperature to effect condensation of at least 25 percent of the CO thus obtaining liquid CO and a hydrogenrich gas mixture containing residual amounts of gaseous C0 0. removing CO from the hydrogen-rich gas mixture to obtain high purity hydrogen, at least part of the CO being removed by absorbing CO at high pressure in a mixed absorbent comprising a chemical absorbent and a physical absorbent;

f. passing the high purity hydrogen to a hydroconversion hydrocarbons and a ho zone wherein hydro en is reacted with hy roconversion zone efpurity hydrogen which comprises:

a. generating by partial oxidation of hydrocarbons at a pressure between psig and about 1200 psig a hydrogen-rich gas containing sufficient CO so that the molecular weight of the hydrogen-rich gas is at least four;

b. centrifugally compressing the hydrogen-rich gas to a substantially higher pressure than said pressure between 100 psig and about 1200 psig, said substantially higher pressure being at least 450 psig, to obtain high pressure hydrogen-rich gas;

0. removing CO from the high pressure hydrogenrich gas to obtain high purity hydrogen, at least part of the CO being removed by absorbing CO in a mixed absorbent comprising a chemical absorbent and a physical absorbent.

12. A process for manufacturing high pressure high purity hydrogen which comprises:

c. removing CO from the high pressure hydrogenrich gas to obtain high purity hydrogen, at least part of the CO being removed by absorbing CO in a mixed absorbent comprising a chemical absorbent and a physical absorbent.

13. A process in accordance with claim 12 wherein the hydrogen is centrifugally compressed after partial removal of CO from the hydrogen-rich gas. 

1. A process for manufacturing high pressure, high purity hydrogen which comprises: a. generating at a pressure below about 450 psig a hydrogen-rich gas containing sufficient CO2 so that the molecular weight of the hydrogen-rich gas is at least four; b. centrifugally compressing the hydrogen-rich gas from a pressure below about 450 psig to a substantially higher pressure above 450 psig to obtain high pressure hydrogen-rich gas; c. removing CO2 from the high pressure hydrogen-rich gas to obtain high purity hydrogen, at least part of the CO2 being removed by absorbing CO2 in a mixed absorbent comprising a chemical absorbent and a physical absorbent.
 2. A process in accordance with claim 1 wherein the chemical absorbent is an alkanolamine.
 3. A process in accordance with claim 1 wherein the chemical absorbent comprises a material selected from the group consisting of monoethanolamine, dipropanolamine and diisopropanolamine, and the physical absorbent comprises a material selected from the group consisting of methanol, acetone, propylene carbonate N-methyl pyrrolidone, tetrahydrothiophene 1,1-dioxide and a dimethyl ether of tetraethylene glycol.
 4. A process in accordance with claim 1 wherein the chemical absorbent comprises an alkanolamine and the physical absorbent comprises tetrahydrothiophene 1,1-dioxide.
 5. A process in accordance with claim 1 wherein the hydrogen-rich gas containing CO2 is generated by steam reforming of hydrocarbons.
 6. A process in accordance with claim 1 wherein the hydrogen-rich gas containing CO2 is generated by partial oxidation of hydrocarbons.
 7. A process in accordance with claim 1 wherein the hydrogen-rich gas at a pressure below about 450 psig is centrifugally compressed to a pressure of at least 900 psig.
 8. In a process for manufacturing high purity hydrogen by a train of steps comprising reacting hydrocarbons with oxygen-affording gas at conditions including a pressure below about 900 psig and elevated temperature to form a gas mixture comprising H2, CO and CO2, reacting CO in the mixture with H2O to form additional H2 and CO2, and removing the CO2 to provide high purity hydrogen, the improvement for providing the high purity hydrogen at elevated pressure which comprises centrifugally compressing the gas mixture comprising H2 and CO2 from the pressure below about 900 psig to a substantially higher pressure above 900 psIg, then removing CO2 from the compressed gas by absorbing CO2 in a mixed absorbent comprising a chemical absorbent and a physical absorbent at a pressure above 900 psig to thus obtain high purity hydrogen.
 9. A process for producing high pressure hydrogen for use in a hydrocarbon hydroconversion zone which comprises the following steps: a. steam reforming the light hydrocarbons at a pressure between one atmosphere and 450 psig to obtain a gas mixture comprised of hydrogen, CO and CO2; b. passing the gas mixture comprising hydrogen, CO and CO2 to at least one shift converter wherein the CO is converted to H2 and CO2 and an effluent is obtained comprised of H2 and CO2; c. compressing the effluent from the shift converter comprising H2 and CO2 in a centrifugal compressor to a pressure of at least 900 psig to obtain a high pressure gas mixture comprised of H2 and CO2; d. removing CO2 from the high pressure gas mixture comprised of H2 and CO2 to obtain high purity hydrogen, at least part of the CO2 being removed by absorbing CO2 at high pressure in a mixed absorbent comprising a chemical absorbent and a physical absorbent; e. passing the high purity hydrogen to a hydroconversion zone wherein hydrogen is reacted with hydrocarbons and a hot hydroconversion zone effluent is obtained; f. passing said hot hydroconversion zone effluent through a steam boiler to generate steam; g. driving said centrifugal compressor using: (1) said steam generated by heat obtained from said hot hydroconversion zone effluent; plus (2) steam generated by heat obtained from said shift converter effluent.
 10. A process for producing high pressure hydrogen for use in a hydrocarbon hydroconversion zone which comprises the following steps: a. steam reforming of light hydrocarbons at a pressure between one atmosphere and 450 psig to obtain a gas mixture comprising hydrogen, CO and CO2; b. passing the gas mixture comprising hydrogen, CO and CO2 to at least one shift converter wherein the CO is converted to H2 and CO2 and an effluent is obtained comprising H2 and CO2; c. compressing the effluent from the shift converter comprising H2 and CO2 in a centrifugal compressor to a pressure of at least 900 psig to obtain a high pressure gas mixture comprising H2 and CO2; d. cooling the high pressure gas mixture comprised of H2 and CO2 to a sufficiently low temperature to effect condensation of at least 25 percent of the CO2, thus obtaining liquid CO2 and a hydrogen-rich gas mixture containing residual amounts of gaseous CO2; e. removing CO2 from the hydrogen-rich gas mixture to obtain high purity hydrogen, at least part of the CO2 being removed by absorbing CO2 at high pressure in a mixed absorbent comprising a chemical absorbent and a physical absorbent; f. passing the high purity hydrogen to a hydroconversion zone wherein hydrogen is reacted with hydrocarbons and a hot hydroconversion zone effluent is obtained; g. passing said hot hydroconversion zone effluent through a steam boiler to generate steam; h. driving said centrifugal compressor using: (1) said steam generated by heat obtained from said hot hydroconversion zone effluent; plus (2) steam generated by heat obtained from said shift converter effluent.
 11. A process for manufacturing high pressure, high purity hydrogen which comprises: a. generating by partial oxidation of hydrocarbons at a pressure between 100 psig and about 1200 psig a hydrogen-rich gas containing sufficient CO2 so that the molecular weight of the hydrogen-rich gas is at least four; b. centrifugally cOmpressing the hydrogen-rich gas to a substantially higher pressure than said pressure between 100 psig and about 1200 psig, said substantially higher pressure being at least 450 psig, to obtain high pressure hydrogen-rich gas; c. removing CO2 from the high pressure hydrogen-rich gas to obtain high purity hydrogen, at least part of the CO2 being removed by absorbing CO2 in a mixed absorbent comprising a chemical absorbent and a physical absorbent.
 12. A process for manufacturing high pressure high purity hydrogen which comprises: a. generating at a pressure below about 450 psig a hydrogen-rich gas containing sufficient CO2 so that the molecular weight of the hydrogen-rich gas is at least four; b. centrifugally compressing the hydrogen from a pressure below about 450 psig to a substantially higher pressure above 450 psig before the molecular weight of the hydrogen-rich gas is reduced below four by CO2 removal, to obtain high pressure hydrogen-rich gas; and c. removing CO2 from the high pressure hydrogen-rich gas to obtain high purity hydrogen, at least part of the CO2 being removed by absorbing CO2 in a mixed absorbent comprising a chemical absorbent and a physical absorbent. 